1. Field of the Invention
This invention relates to forming smooth aluminum films, and more particularly, to a method of depositing aluminum having a subphase of aluminum nitride to produce a hillock-free aluminum film.
2. Description of the Related Art
Metallic films are commonly used to form interconnects on integrated circuits and for display devices such as field emission displays (FEDs). Aluminum is a popular material choice for such films because of its low resistivity, adhesion properties, and mechanical and electrical stability. However, aluminum also suffers from process-induced defects such as hillock formation which may severely limit its performance.
Hillocks are small nodules which form when the aluminum film is deposited or subjected to post-deposition processing. For example, hillocks can result from excessive compressive stress induced by the difference in thermal expansion coefficient between the aluminum film and the underlying substrate used during post-deposition heating steps. Such thermal processing is typical in the course of semiconductor fabrication. Hillock formation may create troughs, breaks, voids and spikes along the aluminum surface. Long term problems include reduced reliability and increased problems with electromigration.
Hillocks may create particularly acute problems in the fabrication of integrated FED and similar devices. Many FEDs comprise two parallel layers of an electrically conductive material, typically aluminum, separated by an insulating layer to create the electric field which induces electron emission. The insulating film is deliberately kept thin (currently about 1-2 xcexcm), to increase the field effect. Hillock formation in the underlying aluminum layer may create spikes through the insulating layer, resulting in a short circuit and complete failure of the device.
Some efforts have been made to reduce or prevent the formation of hillocks in aluminum films. For instance, alloys of aluminum with Nd, Ni, Zr, Ta, Sm and Te have been used to create aluminum alloy thin films which reduce the formation of hillocks. These alloys, however, have been unsatisfactory in producing low resistivity metal lines while still avoiding hillock formation after exposure to thermal cycling.
Accordingly, there is a need for a smooth aluminum film having low resistivity suitable for integrated circuit and field effect display technologies. In particular, the aluminum film should remain hillock-free even after subsequent thermal processing.
The needs addressed above are solved by providing aluminum films, and methods of forming the same, wherein a non-conductive impurity is introduced into the aluminum film. In one embodiment, the introduction of nitrogen creates an aluminum nitride subphase to maintain a substantially smooth surface. The film remains substantially hillock-free even after subsequent thermal processing. The aluminum nitride subphase causes only a nominal increase in resistivity, thereby making the film suitable as an electrically conductive layer for integrated circuit or display devices.
In one aspect of the present invention, a method of forming an electrically conductive metal film for an integrated circuit is provided. The method comprises depositing an aluminum layer onto a substrate assembly, and introducing nitrogen into the aluminum layer while depositing the layer.
In another aspect of the present invention, a method of depositing an aluminum film onto a substrate assembly is provided. The method comprises supplying an inert gas and a nitrogen source gas into a sputtering chamber. The chamber houses the substrate assembly and an aluminum target. The aluminum film is sputtered onto the substrate assembly. In one preferred embodiment, the resultant aluminum film incorporates a sub-phase of aluminum nitride. Exemplary gases introduced into the chamber are Ar and N2. Desirably, H2 is also introduced to further suppress hillock formation in the sputtered film.
In another aspect of the present invention, an electrically conductive aluminum film in an integrated circuit is provided. This film comprises aluminum grains and about 2-10% nitrogen. In one preferred embodiment, the film has a resistivity of between about 5 and 10 xcexcxcexa9cm.
In another aspect of the present invention, a field emission device is provided with a smooth, electrically conductive aluminum layer. The device includes a faceplate and a baseplate, and a luminescent phosphor coating applied to a lower surface of the faceplate to form phosphorescent pixel sites. A cathode member is formed on the baseplate to form individual electron-emission sites which emit electrons to activate the phosphors. The cathode member includes a first semiconductor layer, an emitter tip, an aluminum layer surrounding the tip and incorporating nitrogen, an insulating layer surrounding the tip and overlying the aluminum layer, and a conductive layer overlying the insulating layer.
In another aspect of the present invention, an electrically conductive aluminum wiring element is provided. The film comprises aluminum grains and about 5 to 8% nitrogen in an aluminum nitride subphase. The film has a resistivity of less than about 12 xcexcxcexa9-cm and a surface roughness of less than about 500 xc3x85.